Unify lib/api/merge_requests.rb between CE and EE

What does this MR do?

Unify lib/api/merge_requests.rb between CE and EE

Are there points in the code the reviewer needs to double check?

This doesn't look nice at all, but at least it's working. Could we do better?

CE counterpart: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17539

Why was this MR needed?

Unify CE and EE files so less conflicts.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/4643